The machine around the feature, more than the feature itself: how sophisticated billing needs to be, whether it is multi-tenant B2B software, how many user types and permission levels there are, how much it integrates, and whether it needs an AI layer. The core feature is often the cheap part — billing, tenancy, and admin are where the cost sits.

Because billing is deceptively deep. A subscription is not one feature — it is plans, trials, upgrades, downgrades, proration, failed payments, cancellations, and the account state each implies, all of which must be correct because it touches your revenue. Seat-based B2B billing adds more. It is core infrastructure, not a checkbox.

It builds the core recurring value plus real billing, and defers the rest. It is worth it because the most important thing to learn is whether people will actually pay — and an MVP with working billing lets you find out for £16,000 rather than £60,000. Prove people subscribe, then invest the larger budget in scaling what works.

Third-party usage, passed through at cost and never marked up: payment processing fees, AI inference if the product has an AI layer, and usage-priced infrastructure that scales with your customers. Everything else — design, build, billing setup, the full codebase — is in the fixed price.

Yes. sellyourboat.io is a Wall & Fifth venture that includes a white-label SaaS platform — subscription software we built and run, with 12,000-plus listings, a Claude-powered AI layer, and Stripe billing in production. Having built and operated real SaaS with our own money is why we can price with confidence rather than padding for unknowns.